In a semiconductor device (specifically, a silicon device), by miniaturization (scaling law: Moore's law), integration, power reduction, and the like are advancing at a rate of approximately four times per three years. In recent years, a technology node of a Metal Oxide Semiconductor Field Effect Transistor (MOSFET) has been equal to or smaller than 14 nm, and a lithography process faces a soaring cost (e.g. an apparatus cost and a mask set price), and therewith, a Non-Recurring Cost (NRE) is also increasing. Further, device dimensions are approaching a physical limit (operation limit/variation limit), and high performance of a semiconductor device and high profitability of a semiconductor business are demanded through an approach different from the conventional scaling law.
Among semiconductor devices, as a semiconductor device intermediately positioned between a gate array and a standard cell, a rewritable programmable logic device referred to as a Field Programmable Gate Array (FPGA) has been developed. An FPGA is a semiconductor device settable as any circuit configuration by a user him/herself after fabrication of a chip. Specifically, since an FPGA includes a memory, a user him/herself can optionally connect wirings electrically after fabrication of a semiconductor device. By using a semiconductor device mounted with such an FPGA, a degree of freedom of a circuit can be increased.
In recent years, a chip in which a variable resistance element is used for a memory and a switch of an FPGA has been developed. Examples of such a variable resistance element includes a Resistance Random Access Memory (ReRAM) using a transition metal oxide, a solid electrolyte switch using an ion conductor, an atomic switch, a NanoBridge (Registered Trademark), and the like.
Patent Literature 1 (PTL1) and Non-Patent Literature 1 (NPL1) disclose a configuration, an operation, and a crossbar switch of a two-terminal-type switching element in which two electrodes are disposed via an ion conductor and a conduction state between the two electrodes is controlled.
NPL1 discloses a switching element using migration of metallic ions in an ion conductor and electrochemical reaction. Herein, the ion conductor refers to a conductor in which a carrier is ions and ions in an inside can freely move when an electric field or the like is applied from an outside.
The switching element described in NPL1 has a structure in which two electrodes of a first electrode and a second electrode sandwich a solid electrolyte. Of the two electrodes, the first electrode supplies metallic ions to the solid electrolyte. On the other hand, the second electrode does not supply metallic ions to the solid electrolyte.
Herein, an operation of the switching element described in NPL1 is described.
First, an operation in which the switching element described in NPL1 makes transition from an off-state (high resistance state) to an on-state (low resistance state) is described. In the switching element described in NPL1, when a first electrode is applied with positive voltage while a second electrode is grounded, metal configuring the first electrode is changed to metallic ions and dissolved in a solid electrolyte. Then, the metallic ions are deposited as metal in the sold electrolyte, and a metallic bridge (referred to also as a filament or a conductive path) that electrically connects the first electrode and the second electrode is formed. In other words, the first electrode and the second electrode are electrically connected by a metallic bridge, and thereby the switching element described in NPL1 makes transition from an off-state to an on-state.
In order to cause a switching element to make transition from an on-state to an off-state, a second electrode is applied with positive voltage while a first electrode is grounded. Thereby, a part of a metallic bridge formed in a solid electrolyte is disconnected. Therefore, electric connection between the first electrode and the second electrode is cut off, and therefore the switching element makes transition from an on-state to an off-state. At that time, while an electric characteristic is changed such that an electric resistance between the first electrode and the second electrode is increased or an inter-electrode capacitance is changed from a stage before transition from an on-state to an off-state, the switching element makes transition from an on-state to an off-state. Note that the switching element makes transition again from an off-state to an on-state by applying positive voltage to the first electrode while the second electrode is grounded after electric connection between the first electrode and the second electrode is cut off.
A switching element as described in NPL1 has a feature that a size thereof is smaller than a size of a semiconductor switch (a MOSFET or the like) and an on-resistance (a resistance value in an on-state) is small. Therefore, such a switching element has been thought to be promising for application to a programmable logic device. Further, this switching element maintains a conduction state (on-state) even without being applied with voltage and therefore may be thought to be applied to a non-volatile memory element.
When a switching element is applied to a non-volatile memory, it is conceivable that, for example, a memory cell including one switching element and one selection element such as a transistor is handled as a basic unit. Then, a plurality of these memory cells are arrayed on a substrate in a vertical direction and a lateral direction, respectively, and the memory cells are connected with a word line and a bit line. Memory cells are arrayed in this way, and thereby the word line and the bit line enable any memory cell to be selected from a plurality of memory cells. Thereby, a conduction state of a switching element of the selected memory cell is read, and thereby a non-volatile memory capable of reading any one of pieces of information “1” or “0” according to an on-state or an off-state being stored can be realized.
Patent Literature 2 (PTL2) discloses a variable resistance element in which an electric field concentration effect is generated by forming an electrode shape of a first electrode into a corner shape and thereby program voltage can be reduced and a variation is reduced by stabilizing a position of a metallic bridge deposited from the first electrode.